Support advanced type mappings in the compiled model

[roji]

Starting with 8.0, type mapping must have a static Default property in order to be supported in the compiled model; the generated code starts with these properties, and clones them to overwrite all the different info in RelationalTypeMappingParameters if necessary (store type, CLR type, etc. etc.).

Most type mappings were made compatible with this scheme in #2948. However, one problematic case is when a type mapping has state that isn't contained within RelationalTypeMappingParameters; since the default value can't get overwritten via the standard Clone (with accepts a RelationalTypeMappingParameters), we must customize the C# code generation in RelationalCSharpRuntimeAnnotationCodeGenerator.Create (for type mappings), adding an additional cloning to apply those custom state data.

Type mappings that require this are:

• NpgsqlStringTypeMapping (varchar, xml, citext, ltree as string, lquery, ltxtquery
• NpgsqlULongTypeMapping (xid8)
• NpgsqlRangeTypeMapping
• NpgsqlMultirangeTypeMapping
  • Seems to have some codegen issues (array/list over generic type?), probably on the EF side
• NpgsqlEnumTypeMapping
  • Supported, except where a custom INpgsqlNameTranslator is specified. As the name translator is extracted from the ADO mapping. This is impossible to generate correctly in the compiled model; we can throw specifically for a non-default name translator, otherwise the whole mechanism needs to be rethought (I want to do that for other reasons).

/cc @AndriySvyryd